Cullen stated he administered overdoses to patients in order to spare them from being "coded" — going into cardiac or respiratory arrest and being listed as a Code Blue emergency. He told detectives that he could not bear witness to or hear about attempts at saving a victim's life. Cullen also stated that he gave patients overdoses so that he could end their suffering and prevent hospital personnel from dehumanizing them. However, not all of his victims were terminal patients. Some, like Gall, had been expected to recover before Cullen killed them. Nurse Lynn Tester described many of the victims as "people on the mend" in a police interview.
Instead of using common painkillers and stimulants, access of which was strictly regulated by hospitals due to their value as street drugs, Cullen chose instead to use as his tools of choice drugs such as digoxin and insulin which had little use outside of a hospital setting and were less likely to attract attention.
Investigators stated that Cullen may have caused patients to suffer, but that he appears not to realize this, contradicting his claims of wanting to save patients. Similarly, Cullen told investigators that although he often observed patients' suffering for several days, the decision to commit each murder was performed on impulse. Cullen told detectives in December 2003 that he lived most of his life in a fog and that he had blacked out memories of murdering most of his victims. He said he could not recall how many he killed or why he had chosen them. In some cases, Cullen adamantly denied committing any murders at a given facility, but after reviewing medical records, he admitted that he was involved in patient deaths.
Cullen was arrested at a restaurant on December 12, 2003, charged with one count of murder and one count of attempted murder. On December 14, he admitted to homicide detectives Dan Baldwin and Tim Braun that he had murdered Rev. Florian Gall and attempted to murder Jin Kyung Han, both patients at Somerset. In addition, Cullen told the detectives that he had murdered as many as forty patients over his sixteen-year career. In April 2004, Cullen pled guilty in a New Jersey court to killing thirteen patients and attempting to kill two others by lethal injection while employed at Somerset.
As part of his plea agreement, he promised to cooperate with authorities if they did not seek the death penalty for his crimes. A month later, he pled guilty to the murder of three more patients in New Jersey. In November 2004, Cullen pled guilty in an Allentown court to killing six patients and trying to kill three others. He repeatedly interrupted the proceedings by taunting the judge with the chant "Your Honor, you need to step down." Cullen was ordered to be restrained and gagged, but it did not stop him.
On March 2, 2006, Cullen was sentenced to eleven consecutive life sentences in New Jersey, and is not eligible for parole until year 2403. Currently, he is held at New Jersey State Prison in Trenton. On March 10, Cullen was brought into the courtroom of Lehigh County President Judge William H. Platt for a sentencing hearing. Cullen, upset with the judge, kept repeating, "Your honor, you need to step down" for thirty minutes until Platt had Cullen gagged with cloth and duct tape. Even after being gagged, Cullen continued to try to repeat the phrase.
In this hearing, Platt gave him an additional six life sentences. As part of his plea agreement, Cullen has been working with law enforcement officials to identify additional victims. He is currently serving a sentence of life in prison without parole for over 100 years, to be served consecutively with his other sentences in Pennsylvania.